diff --git a/modules/order/src/Form/CustomerFormTrait.php b/modules/order/src/Form/CustomerFormTrait.php
index 422bbfbe..a5ad3c41 100644
--- a/modules/order/src/Form/CustomerFormTrait.php
+++ b/modules/order/src/Form/CustomerFormTrait.php
@@ -5,7 +5,6 @@ namespace Drupal\commerce_order\Form;
 use Drupal\commerce_order\Entity\OrderInterface;
 use Drupal\Component\Utility\Html;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\user\Entity\User;
 
 /**
  * Provides a form for selecting the order's customer (uid and mail fields).
@@ -14,6 +13,13 @@ use Drupal\user\Entity\User;
  */
 trait CustomerFormTrait {
 
+  /**
+   * The user storage.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageInterface
+   */
+  protected $userStorage;
+
   /**
    * Builds the customer form.
    *
@@ -123,25 +129,45 @@ trait CustomerFormTrait {
   }
 
   /**
-   * Submit handler for the customer select form.
+   * Validation handler for the customer select form.
    *
    * @param array $form
    *   The parent form.
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current state of the form.
    */
-  public function submitCustomerForm(array &$form, FormStateInterface $form_state) {
+  public function validateCustomerForm(array &$form, FormStateInterface $form_state) {
     $values = $form_state->getValues();
-    if ($values['customer_type'] == 'existing') {
-      $values['mail'] = User::load($values['uid'])->getEmail();
-    }
-    else {
-      $user = User::create([
+    if (isset($values['mail']) && $values['customer_type'] == 'new') {
+      $user = $this->userStorage->create([
         'name' => $values['mail'],
         'mail' => $values['mail'],
         'pass' => ($values['generate']) ? user_password() : $values['pass'],
         'status' => TRUE,
       ]);
+      $form_state->set('customer', $user);
+      $violations = $user->validate();
+      foreach ($violations as $violation) {
+        $form_state->setErrorByName(str_replace('.', '][', $violation->getPropertyPath()), $violation->getMessage());
+      }
+    }
+  }
+
+  /**
+   * Submit handler for the customer select form.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the parent form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  public function submitCustomerForm(array &$form, FormStateInterface $form_state) {
+    $values = $form_state->getValues();
+    if ($values['customer_type'] == 'existing') {
+      $values['mail'] = $this->userStorage->load($values['uid'])->getEmail();
+    }
+    else {
+      $user = $form_state->get('customer');
       $user->save();
       $values['uid'] = $user->id();
 
diff --git a/modules/order/src/Form/OrderAddForm.php b/modules/order/src/Form/OrderAddForm.php
index 9e1e9215..239d16a4 100644
--- a/modules/order/src/Form/OrderAddForm.php
+++ b/modules/order/src/Form/OrderAddForm.php
@@ -30,6 +30,13 @@ class OrderAddForm extends FormBase {
    */
   protected $storeStorage;
 
+  /**
+   * The user storage.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageInterface
+   */
+  protected $userStorage;
+
   /**
    * Constructs a new OrderAddForm object.
    *
@@ -39,6 +46,7 @@ class OrderAddForm extends FormBase {
   public function __construct(EntityTypeManagerInterface $entity_type_manager) {
     $this->orderStorage = $entity_type_manager->getStorage('commerce_order');
     $this->storeStorage = $entity_type_manager->getStorage('commerce_store');
+    $this->userStorage = $entity_type_manager->getStorage('user');
   }
 
   /**
@@ -115,6 +123,13 @@ class OrderAddForm extends FormBase {
     return $form;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $this->validateCustomerForm($form, $form_state);
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/modules/order/src/Form/OrderReassignForm.php b/modules/order/src/Form/OrderReassignForm.php
index d1b9de40..754e57b7 100644
--- a/modules/order/src/Form/OrderReassignForm.php
+++ b/modules/order/src/Form/OrderReassignForm.php
@@ -115,6 +115,13 @@ class OrderReassignForm extends FormBase {
     return $form;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $this->validateCustomerForm($form, $form_state);
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/modules/order/tests/src/FunctionalJavascript/OrderAdminTest.php b/modules/order/tests/src/FunctionalJavascript/OrderAdminTest.php
index 864e6c98..ef4ac164 100644
--- a/modules/order/tests/src/FunctionalJavascript/OrderAdminTest.php
+++ b/modules/order/tests/src/FunctionalJavascript/OrderAdminTest.php
@@ -463,4 +463,23 @@ class OrderAdminTest extends OrderWebDriverTestBase {
     $this->assertSession()->pageTextContains('Access denied');
   }
 
+  /**
+   * Tests creating a new customer with same email used by an existing customer.
+   */
+  public function testCreateOrderNewCustomer() {
+    $email = 'guest@example.com';
+    $this->createUser(['mail' => $email]);
+
+    $this->drupalGet('/admin/commerce/orders');
+    $this->getSession()->getPage()->clickLink('Create a new order');
+    $this->getSession()->getPage()->selectFieldOption('customer_type', 'new');
+    $this->assertSession()->assertWaitOnAjaxRequest();
+    $edit = [
+      'customer_type' => 'new',
+      'mail' => $email,
+    ];
+    $this->submitForm($edit, t('Create'));
+    $this->assertSession()->pageTextContains('The email address guest@example.com is already taken.');
+  }
+
 }